Is there a saying or word for indicating the overuse of something you just newly learned? Say you were happy with a hammer and a nail and then somebody taught you the virtues of a screw and screwdriver. From then on, every problem incorrectly looks like it can be solved only with the screwdriver.

Yes - or, to paraphrase Maslow, When you've just learned how to use a hammer, everything looks like a nail
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FumbleFingersJan 27 '13 at 17:15

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@FumbleFingers: You should post this comment as an answer--the whole law of the instrument concept. Nice find.
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CallithumpianJan 27 '13 at 17:17

On reflection, I think I probably should. It doesn't inherently imply using a recently-acquired method/tool, but in practice it would often be used in such contexts.
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FumbleFingersJan 27 '13 at 17:20

@Cerb: Wow! 58 upvotes and counting! Definitely worth posting as an answer. (We should ask the mods to transfer you half-a-dozen of those upvotes as a 10% "agent's fee"! :)
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FumbleFingersJan 30 '13 at 15:56

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@FumbleFingers: My normal rate is 20% but I'll give you a discount this time since I've also benefitted from the extra traffic to this question. Just make sure they're transferred to me and not "Cerb." :)
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CallithumpianJan 30 '13 at 17:04

A slightly oblique suggestion: the Baader-Meinhof phenomenon. It's also known as the recency syndrome, and it describes the situation where you suddenly begin to notice X everywhere moments/days after you first learn about X. To quote people smarter than me: "the belief that things you started noticing recently are actually recent".

The most common example of this is how you seem to suddenly hear a specific song everywhere after someone introduces it to you or calls your attention to it. The song had probably been playing just as frequently on the radio / in stores both before and after you were introduced to the song, but now that your attention has been called to it, your brain picks up new instances of the song being played much more readily than it did before. It's a case of selective attention.

So, to tie this back to your question: you might encounter X events per day that call for the need of a hammer, but before someone gave you one as a present you just glossed over those events, and after someone gave you a hammer you start consciously noticing places you can use it, so that it seems like there's suddenly so much more opportunity to use your shiny new toy.

I like to use the general phrase “[noun] evangelist” because I tend to believe anyone who gets truly hyped about something new falls into that unique—and highly annoying—type of human behavior where they become a pseudo-religious zealot about the topic.

I understand that guy’s POV on web standards, but he is such an HTML5evangelist he makes me want to start using Adobe PageMill 1.0 again.

Another method of describing such behavior I use is:

He is so zealous about the topic he sucks any shred of joy
I might might have gotten out of the topic now or ever again.

I cannot find a good reference, I first heard it on an episode of The West Wing. While it seems that people use it (from google hits) I can't seem to find a decent discussion of the phrase. Still its meaning is rather clear.